The present invention relates generally to a phase change memory device, and more particularly, to a phase change memory device having decentralizing drivers for driving word lines to a ground voltage to minimize the resistance in the word line.
Phase change memory (PRAM) devices are a type of nonvolatile memory currently being developed and gaining in popularity. Since the PRAM is nonvolatile, it conserves data even after the power is turned off.
A unit cell of the PRAM device uses a phase change material as a data storage medium. The phase change material has two stable states (amorphous phase and crystalline phase), and the phase of the material depends on the heat applied to the material. Typically, the phase change material is GST (Ge-Sb-Te), which is a compound of germanium (Ge), stibium (Sb), and tellurium (Te).
The GST phase change material becomes amorphous when the GST is heated close to the melting point of the GST for a short period of time and then rapidly cooled. Conversely, the GST phase change material becomes crystalline when the GST is heated at a crystallization temperature lower than the melting point for a long period of time and then slowly cooled. The amorphous GST has a higher resistance than that of the crystalline GST. Using this phenomenon, current flowing through the phase change material is sensed, making it possible to judge whether information stored in the cell of the PRAM is data logic value “1” (amorphous phase) or a data logic value “0” (crystalline phase).
In the PRAM device, Joule heating is performed on the phase change material. That is, current is supplied to an electrode connected to the phase change material. The current supplied to the electrode generates heat, and the heat is supplied to the phase change material. The amount of the heat supplied to the phase change material depends on the amount of the supplied current.
FIG. 1 is a circuit diagram showing a unit phase change resistance cell.
The unit phase change resistance cell C includes a phase change resistor PCR and a diode D. The PCR has a first electrode connected to a word line WL and a second electrode connected to an N-type region of the diode D. The P-type region of the diode D is connected to the bit line BL, and the N-type region is connected to the word line WL. Current flows through the PCR when the bit line BL becomes a high level and the word line WL becomes a low level. At this time, the PCR can be changed to a crystalline or amorphous phase according to the current flowing through the bit line BL.
FIG. 2 is a diagram showing a phase change memory device.
The phase change memory device comprises a cell array CA, a word line driving unit 10, a decoding unit 20, and a bit line driving unit 30.
The cell array CA includes a plurality of bit lines BL0˜BLn arranged in a column direction and a plurality of word lines WL0˜WL3 arranged in a row direction. The cell array CA includes a plurality of unit phase change resistance cells C each of which is positioned at the intersection of one of the bit lines BL0˜BLn and one of the word lines WL0˜WL3.
The word line driving unit 10 includes a plurality of drivers each configured to drive an output signal of the decoding unit 20 in order to supply the output signal to the word lines WL0˜WL3. The driver includes a plurality of inverters IV1˜IV4 each of which is connected to one of the word lines WL0˜WL3. The inverter IV1 inverts the output signal of the decoding unit 20 and outputs the inverted signal to the word line WL0. The inverter IV2 inverts the output signal of the decoding unit 20 and outputs the inverted signal to the word line WL1. The inverter IV3 inverts the output signal of the decoding unit 20 and outputs the inverted signal to the word line WL2. The inverter IV4 inverts the output signal of the decoding unit 20 and outputs the inverted signal to the word line WL3.
The decoding unit 20 decodes a row address and outputs the decoded row address. The bit line driving unit 30 supplies a write voltage corresponding to the data state of the bit line BL.
When in write mode, the selected bit line BL becomes a high level, and the selected word line WL becomes a low level. Therefore, a current path is formed from the bit line BL to the word line WL. The current for writing data depends on the structure of the word line WL; and therefore, the resistance Rwl in the word line WL is required to be small, and the electric potential of the word line WL is required to be at a ground voltage VSS level.
However, the resistance Rwl in the word line WL increases as the length of the word line WL becomes longer and the distance between word lines WL becomes narrower. The larger resistance Rwl causes a reduction in the current flowing through the PCR, and as a consequence data cannot be written normally.
Additionally, in this case, an increased loading time of the word line WL requires a larger driving unit 10 so that word line WL can be driven to the ground voltage VSS level. However, with a highly integrated device the distance between the word lines WL is narrower limiting the ability to form a large driving unit 10. Moreover, when a decoding unit is added, the chip size increases.